1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method which perform printing by ejecting an ink from a plurality of ejection ports to a printing medium. In detail, the present invention relates to an ink jet printing apparatus and an ink jet printing method which perform printing using a printing head equipped with a plurality of ejection port arrays ejecting the same color ink.
2. Description of the Related Art
A printer or a copy machine and the like, a printing apparatus used as an output device for composite electronics, a work station including a computer or a word processor is configured so that printing can be performed on a printing medium such as paper or a plastic thin sheet based on printing information. The printing apparatus like this is classified into an ink jet type, a wire dot type, a thermal type, a laser beam type, or the like. The printing apparatus of the ink jet type among printing apparatuses of such various printing types uses an ink jet printing head (hereinafter, referred to also as a printing head) as a printing means to perform printing by ejecting an ink toward the printing medium from an ejection port provided in the printing head. The printing apparatus of such ink jet type (hereinafter, referred to also as an ink jet printing apparatus) has advantages that the printing head is easily downsized, that high resolution image can be formed rapidly, and that noise is small because of non-impact type.
The ink jet printing apparatus like this is roughly classified into two types of a serial type and a full line type depending on its printing method. The ink jet printing apparatus of the serial type uses a method to perform printing while scanning a printing head in a main scanning direction intersecting with a conveying direction of the printing medium (sub scanning direction). In this method, every time a printing movement in one time main scanning is finished, a movement in which the printing medium is conveyed by a predetermined amount is performed, and the printing on all regions of the printing medium is performed by repeating the printing movement and the conveyance of the printing medium. On the other hand, the ink jet printing apparatus of the full line type uses a printing method to perform only a movement of the printing medium in the conveying direction upon printing. In the full line type, the printing on all regions of the printing medium is performed by performing printing continuously for one line while conveying the printing medium by use of the printing head in which ejection ports are arranged across the entire width of the printing medium. The ink jet printing apparatus of such full line type uses a printing method having a capability of printing with higher speed in comparison with the serial type. For example, the printing with a resolution of 600×600 dpi (dot/inch) for the printing of mono-color such as a sentence, or a high resolution printing with a resolution of 1200×1200 dpi or more for the printing of full-color picture like a photon can be also performed at a high speed of 60 pages or more per minute on the printing medium sized A4.
In the ink jet printing apparatus of the full line type, each of the ejection ports that are arranged across the full width of printing region prints dots arranged along the conveying direction (a direction intersecting with this conveying direction is referred to as the main scanning direction hereinafter). Accordingly, as with so-called multi-path printing which performs one line printing with a plurality of scannings in the serial type, one line is printed with a plurality of ejection ports, therefore, a variation of ejecting characteristic between the ejection ports cannot be reduced. Because of this, when the ejecting characteristic has a variation such that ejecting is not performed normally, and that an impact location displaces, this type has a defect that a fault in the printing such as stripe or stripe unevenness may easily appear. Originally, it is to be desired that all ejection ports shall be manufactured with no defect and excellent accuracy. However, the number of the ejection ports is great; therefore, it is very hard to manufacture them with no defect and excellent accuracy. For example, for performing the printing with the resolution of 1200 dpi in a sheet sized A3, it is necessary to provide about fourteen thousand units of the ejection ports (printing width 297 mm) in the printing head of the full line type. Therefore, if they can be manufactured, manufacturing cost tends to increase because the non-defective ratio is low. Because of this, in the printing head of the full line type, a constitution of so-called connection heads so as to realize a long head by arranging relatively low cost short heads used for the printing of the serial type is commonly constructed in such a manner that a plurality of units are connected in an arrangement direction of the ejection ports.
As one constitution reducing a problem of the above-mentioned variation caused by the printing head of the full line type, in order to weaken an influence applied to the printing with one ejection port, a constitution in which dots on one line along the main scanning direction shall be printed by not one ejection port but a plurality of ejection ports is employed. This multi-array constitution of the ejection port arrays can realize printing of a high-quality picture by reducing the variation of the ejecting characteristic between the ejection ports as well as a multi-path printing in the printing of the serial type. For example, a picture quality of the same level as 4-path printing in the printing of the serial type can be realized in such a way that the ejection port array is constituted to be multiple as with a constitution in which 4-array ejection ports per one color are provided to be shifted in the conveying direction of the printing medium.
However, the present inventors examined and revealed that, when the printing is performed using the printing head of the multi-array constitution like this, uneven thickness with density varied with respect to the main scanning direction, so called conveyance unevenness tends to occur. Specifically, when the plural ejection port arrays arranged in a direction intersecting with the main scanning direction at approximately right angles are arranged with a certain distance in the conveying direction of the printing medium, it is found that the conveyance unevenness occurs remarkably as the distance between those ejection port arrays becomes great. This is caused by a phenomenon in which the printing medium may be conveyed meanderingly. At that time, the uneven thickness may occur in such a way that the impact location displaces depending on a difference of eject timing between the ejection port arrays.
FIG. 16 is a drawing illustrating a situation performing the printing on a printing medium 5 conveyed in the arrow direction in the drawing with a printing head of 4-array constitution (array A, array B, array C, and array D) for the same ink color. Further, FIG. 17 is a graph showing a printing displacement (hereinafter, also referred to as X displacement) caused in such a manner that the printing medium is conveyed meanderingly in a state like a sine curve when the printing is performed with the printing head shown in FIG. 16.
As is apparent from FIG. 16, each of four ejection port arrays is arranged mutually in parallel with a fixed interval in the main scanning direction. In addition, a row direction of four ejection port arrays is equivalent to the conveying direction of the printing medium. Accordingly, when the printing is performed with ejection ports of four ejection port arrays, printing timing is different for each array. Incidentally, a dot of the same color is not printed to be overlapped so often at the same location of the printing medium. Normally, the dots are printed in order with four ejection ports so that they may be adjacent in the main scanning direction with a pitch depending on the resolution. However, since a mutual spacing between these four ejection port arrays is far greater than the pitch of the above-mentioned adjacent dots, hereinafter, a location at which the dots are printed adjacently in the main scanning direction with these plural ejection ports is described as the same location for simplified description. When the printing is performed at the same location like this, ejection timing is different for each ejection port array, and a printing displacement of each ejection port array caused by the difference leads to a condition that phase is shifted as shown in FIG. 17.
A relation between a graph in FIG. 17 and a result of the printing will be described. In any graph of the arrays from the array A to the array D, there is X-displacement within a range from +15 μm to −15 μm so as to draw a sine (sine wave) curve, and the phase is shifted by the amount corresponding to the difference in ejection timing. Regarding printing result, the printing result of the case in which a straight line is drawn without displacement in X is most preferable, and the uneven thickness does not occur either.
By the way, a portion in which a difference of X displacement among ejection port arrays in each graph shown in FIG. 17 is small is each of inflection points of Q1, Q2, Q3, and Q4, and the printing results equivalent to portions near these inflection points Q1, Q2, Q3, and Q4 give almost excellent printing results. Further, in portions except the inflection points, namely, notwithstanding from plus to minus or from minus to plus, P1, P2, P3, and P4 which are large in X displacement variation amount, the printing becomes rough as a result that the impact location of the ink ejected is displaced. Accordingly, the printing result becomes a result with prominent uneven thickness in which dense portion and rough portion are generated alternately.
FIG. 18 shows that a difference of the X displacement between the array A and the array D and a difference of the X displacement between the array A and the array B in each main scanning position in FIG. 17 are represented in a graph. The comparison of FIG. 17 and FIG. 18 shows that the difference of the X displacement becomes small at a portion equivalent to the inflection points Q1, Q2, Q3, and Q4 in FIG. 17. The comparison also shows that the difference between the array A and the array B, which are short in distance between the ejection port arrays is smaller than the difference between the array A and the array D, which are long in distance between the ejection port arrays. Namely, the shorter the distance between the ejection port arrays becomes, the less the uneven thickness becomes. Inversely, since the longer the distance between the ejection port arrays becomes, the greater the X displacement becomes, the uneven thickness is generated remarkably accordingly. In particular, in a photographic output in which high image quality is required, the uneven thickness like this becomes an unacceptable level.
As mentioned above, the shorter the distance between the ejection port arrays becomes, the less the uneven thickness becomes. Namely, the uneven thickness generated in the printing result can be normally eliminated by performing the printing with one ejection port array. However, in this case, an effect of so called multi-array constitution, in which when a certain ejection port has a failure of miss ejecting, other ejection port performs supplemental ejecting, can not be obtained, therefore, the printing result with high quality printing can not be obtained.
By the way, the uneven thickness generated in the printing result is conspicuous in the half tone portion in particular. Since the half tone portion has a gradation in which the dots impacted per a unit area are contacted or overlapped each other, when the impact location displaces, a variation of covering ratio (called [area factor] also) of the ink with respect to the unit area of the printing medium is greater in comparison with that of the other gradation. Therefore, the impacted dot with displacement is likely to be visible. As compared with the above, since the dots are separately arranged normally in a portion in which the number of impacting dots per the unit area is small, the variation of the covering ratio is hard to occur even when the impact location displaces. On the other hand, since the dots are densely impacted being mutually overlapped in a portion in which the number of the impacting dots per the unit area is large, the variation of the covering ratio is hard to occur because an influence of the impact location displacement is hard to be received.
Incidentally, a meandering in the printing medium conveyance causing the above-mentioned problem, needless to say, needs not be a complete sine wave curve as mentioned above. Further, even when the meandering is generated in a part of the conveyance, it is evident that the above-mentioned problem is caused in that part.
Furthermore, the above-mentioned uneven thickness can be thought to be naturally eliminated by suppressing a conveyance deviation of the printing medium as much as possible. However, the deviation generated on the apparatus like this is hard to be eliminated completely. Therefore, the displacement of several 10 μm or so tends to be generated while conveying the printing medium. On the other hand, as the distance between the plural ejection port arrays is made to be shortened relatively, the uneven thickness becomes not conspicuous because a location displacement influence of the impacting is reduced. However, the distance between the ejection port arrays is hard to be shortened from a consideration of arrangement of the ejection port, a wiring layout of the printing element provided in the ejection port, securement of a space portion in which the ink jet printing head and a cap protecting the ink jet printing head may contact each other, and the like.